Public Advocate Bill de Blasio and UFT President Michael Mulgrew spoke out against the release of Teacher Data Reports outside P.S. 321 in Brooklyn Monday morning.

UFT President Michael Mulgrew started his week at P.S. 321, a high-performing elementary school in Park Slope whose principal has taken an unusually outspoken stance against the release of thousands of individual teachers’ city ratings.

Elizabeth Phillips, the school’s longtime principal, published a column on the New York City Public School Parents blog this weekend arguing that the Teacher Data Reports were based on inaccurate data and generated results that conflicted with her own assessments’ of teachers.

The reports are years-old “value-added” assessments of teacher effectiveness for about 18,000 city teachers who taught math and reading in grades 4-8 between 2007 and 2010. They were released Friday after a long legal fight, and many local news organizations chose to publish them. GothamSchools did not because of concerns about the data.

Dick Riley, a union spokesman, said P.S. 321 had been chosen for Mulgrew’s appearance because it was a successful school that was accessible for reporters. That Phillips had taken a strong stance against publication was “serendipitous,” he said.

Standing outside the school as teachers and families started to trickle in, Mulgrew said the reports’ release was potentially a watershed moment for city teachers.

“We’re going to do everything in our power to prevent the mayor doing any more damage to the city’s schools,” he told reporters. The comment echoed one he made to the New York Times, which reported today that the release could wind up being a political win for the union by galvanizing support at a time when Mayor Bloomberg and others have taken aim at the union and its members.

Today, Mulgrew told GothamSchools, “More and more teachers are becoming more motivated to really start pushing against this mayor.”

Mulgrew said he had spent the weekend speaking with teachers who were concerned about repercussions of having their ratings in the public view for the first time. One teacher whose low score was mentioned in the press this weekend told him that she had chosen to work with some of the city’s highest-need students and had gotten positive reviews from her principal but now feels demoralized. “I don’t even want to leave my house anymore,” Mulgrew said the teacher told him.

Teachers entering P.S. 321 — a school that is bursting at the seams with Park Slope families who are eager to enroll — said they were also unsettled by the ratings’ release.

Simone Frasier, a 12-year fourth-grade teacher, called the release “outrageous and disheartening.” Ronda Matthews, a fifth-grade teacher, said, “It’s another way to be humiliated as a teacher. It absolutely has no recognition of what we do all day.”

Both teachers said they had received reports but declined to discuss them. “It’s irrelevant,” Matthews said.

Some of P.S. 321′s teachers’ scores were likely affected by added instability among teachers whose students mostly had very high or very low scores. The difference is due to the nature of the state tests, which were designed to distinguish among middle-level students — those just above and below the state’s proficiency cutoff. As a result, small differences among students at the top and bottom had an outsized impact on a teacher’s rating.

Phillips, who emerged from the building to greet Mulgrew and Public Advocate Bill de Blasio, said the large margins of error for teachers of high-performing students rendered P.S. 321′s scores useless. Last year, 87 percent of the school’s students passed the state’s reading exams and 92 percent passed the math tests.

“When you’re talking about high-performing kids, whether a child gets two questions wrong in third grade and three questions wrong in fourth grade — it’s meaningless and goofy,” she said.

But Phillips said the release would have been just as “devastating and demoralizing” to teachers even if the underlying test scores and formula were sound.

“I don’t think it’s appropriate to publish any test scores with teachers’ names attached even if it’s done correctly,” she said.

Earlier this month, de Blasio urged the state and city to sign on to new teacher evaluations, which would be based in part on a value-added analysis of student performance similar to the one the city applied to generate the Teacher Data Reports. But de Blasio, a presumptive candidate for next year’s mayoral race, said the public could learn enough from seeing school- or district-wide composite scores.

“Releasing names — I still think it’s problematic,” he said. “I don’t think it’s necessarily the best way to manage a system.”

For their part, P.S. 321 parents said they weren’t putting much stock in the ratings.

Cassie Schwerner, a parent who works for an education policy foundation, said the ratings would get “less than zero” consideration from her family, which she said had made a joint decision not to seek out teachers’ scores. Another father bringing his daughter to school said he hadn’t heard much about the ratings this weekend and wouldn’t seek them out now.